.\" $OpenBSD: ssh-agent.1,v 1.79 2023/08/10 14:37:32 naddy Exp $
.\"
.\" Author: Tatu Ylonen <ylo@cs.hut.fi>
.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
.\"                    All rights reserved
.\"
.\" As far as I am concerned, the code I have written for this software
.\" can be used freely for any purpose.  Any derived versions of this
.\" software must be clearly marked as such, and if the derived work is
.\" incompatible with the protocol description in the RFC file, it must be
.\" called by a name other than "ssh" or "Secure Shell".
.\"
.\" Copyright (c) 1999,2000 Markus Friedl.  All rights reserved.
.\" Copyright (c) 1999 Aaron Campbell.  All rights reserved.
.\" Copyright (c) 1999 Theo de Raadt.  All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
.\"
.Dd $Mdocdate: August 10 2023 $
.Dt SSH-AGENT 1
.Os
.Sh NAME
.Nm ssh-agent
.Nd OpenSSH authentication agent
.Sh SYNOPSIS
.Nm ssh-agent
.Op Fl c | s
.Op Fl \&Dd
.Op Fl a Ar bind_address
.Op Fl E Ar fingerprint_hash
.Op Fl O Ar option
.Op Fl P Ar allowed_providers
.Op Fl t Ar life
.Nm ssh-agent
.Op Fl a Ar bind_address
.Op Fl E Ar fingerprint_hash
.Op Fl O Ar option
.Op Fl P Ar allowed_providers
.Op Fl t Ar life
.Ar command Op Ar arg ...
.Nm ssh-agent
.Op Fl c | s
.Fl k
.Sh DESCRIPTION
.Nm
is a program to hold private keys used for public key authentication.
Through use of environment variables the agent can be located
and automatically used for authentication when logging in to other
machines using
.Xr ssh 1 .
.Pp
The options are as follows:
.Bl -tag -width Ds
.It Fl a Ar bind_address
Bind the agent to the
.Ux Ns -domain
socket
.Ar bind_address .
The default is
.Pa $TMPDIR/ssh-XXXXXXXXXX/agent.\*(Ltppid\*(Gt .
.It Fl c
Generate C-shell commands on
.Dv stdout .
This is the default if
.Ev SHELL
looks like it's a csh style of shell.
.It Fl D
Foreground mode.
When this option is specified,
.Nm
will not fork.
.It Fl d
Debug mode.
When this option is specified,
.Nm
will not fork and will write debug information to standard error.
.It Fl E Ar fingerprint_hash
Specifies the hash algorithm used when displaying key fingerprints.
Valid options are:
.Dq md5
and
.Dq sha256 .
The default is
.Dq sha256 .
.It Fl k
Kill the current agent (given by the
.Ev SSH_AGENT_PID
environment variable).
.It Fl O Ar option
Specify an option when starting
.Nm .
Currently two options are supported:
.Cm allow-remote-pkcs11
and
.Cm no-restrict-websafe .
.Pp
The
.Cm allow-remote-pkcs11
option allows clients of a forwarded
.Nm
to load PKCS#11 or FIDO provider libraries.
By default only local clients may perform this operation.
Note that signalling that an
.Nm
client is remote is performed by
.Xr ssh 1 ,
and use of other tools to forward access to the agent socket may circumvent
this restriction.
.Pp
The
.Cm no-restrict-websafe
option instructs
.Nm
to permit signatures using FIDO keys that might be web authentication
requests.
By default,
.Nm
refuses signature requests for FIDO keys where the key application string
does not start with
.Dq ssh:
and when the data to be signed does not appear to be a
.Xr ssh 1
user authentication request or a
.Xr ssh-keygen 1
signature.
The default behaviour prevents forwarded access to a FIDO key from also
implicitly forwarding the ability to authenticate to websites.
.It Fl P Ar allowed_providers
Specify a pattern-list of acceptable paths for PKCS#11 provider and FIDO
authenticator middleware shared libraries that may be used with the
.Fl S
or
.Fl s
options to
.Xr ssh-add 1 .
Libraries that do not match the pattern list will be refused.
See PATTERNS in
.Xr ssh_config 5
for a description of pattern-list syntax.
The default list is
.Dq usr/lib*/*,/usr/local/lib*/* .
.It Fl s
Generate Bourne shell commands on
.Dv stdout .
This is the default if
.Ev SHELL
does not look like it's a csh style of shell.
.It Fl t Ar life
Set a default value for the maximum lifetime of identities added to the agent.
The lifetime may be specified in seconds or in a time format specified in
.Xr sshd_config 5 .
A lifetime specified for an identity with
.Xr ssh-add 1
overrides this value.
Without this option the default maximum lifetime is forever.
.It Ar command Op Ar arg ...
If a command (and optional arguments) is given,
this is executed as a subprocess of the agent.
The agent exits automatically when the command given on the command
line terminates.
.El
.Pp
There are two main ways to get an agent set up.
The first is at the start of an X session,
where all other windows or programs are started as children of the
.Nm
program.
The agent starts a command under which its environment
variables are exported, for example
.Cm ssh-agent xterm & .
When the command terminates, so does the agent.
.Pp
The second method is used for a login session.
When
.Nm
is started,
it prints the shell commands required to set its environment variables,
which in turn can be evaluated in the calling shell, for example
.Cm eval `ssh-agent -s` .
.Pp
In both cases,
.Xr ssh 1
looks at these environment variables
and uses them to establish a connection to the agent.
.Pp
The agent initially does not have any private keys.
Keys are added using
.Xr ssh-add 1
or by
.Xr ssh 1
when
.Cm AddKeysToAgent
is set in
.Xr ssh_config 5 .
Multiple identities may be stored in
.Nm
concurrently and
.Xr ssh 1
will automatically use them if present.
.Xr ssh-add 1
is also used to remove keys from
.Nm
and to query the keys that are held in one.
.Pp
Connections to
.Nm
may be forwarded from further remote hosts using the
.Fl A
option to
.Xr ssh 1
(but see the caveats documented therein),
avoiding the need for authentication data to be stored on other machines.
Authentication passphrases and private keys never go over the network:
the connection to the agent is forwarded over SSH remote connections
and the result is returned to the requester,
allowing the user access to their identities anywhere in the network
in a secure fashion.
.Sh ENVIRONMENT
.Bl -tag -width "SSH_AGENT_PID"
.It Ev SSH_AGENT_PID
When
.Nm
starts, it stores the name of the agent's process ID (PID) in this variable.
.It Ev SSH_AUTH_SOCK
When
.Nm
starts, it creates a
.Ux Ns -domain
socket and stores its pathname in this variable.
It is accessible only to the current user,
but is easily abused by root or another instance of the same user.
.El
.Pp
In Debian,
.Nm
is installed with the set-group-id bit set, to prevent
.Xr ptrace 2
attacks retrieving private key material.
This has the side-effect of causing the run-time linker to remove certain
environment variables which might have security implications for set-id
programs, including
.Ev LD_PRELOAD ,
.Ev LD_LIBRARY_PATH ,
and
.Ev TMPDIR .
If you need to set any of these environment variables, you will need to do
so in the program executed by ssh-agent.
.Sh FILES
.Bl -tag -width Ds
.It Pa $TMPDIR/ssh-XXXXXXXXXX/agent.<ppid>
.Ux Ns -domain
sockets used to contain the connection to the authentication agent.
These sockets should only be readable by the owner.
The sockets should get automatically removed when the agent exits.
.El
.Sh SEE ALSO
.Xr ssh 1 ,
.Xr ssh-add 1 ,
.Xr ssh-keygen 1 ,
.Xr ssh_config 5 ,
.Xr sshd 8
.Sh AUTHORS
.An -nosplit
OpenSSH is a derivative of the original and free ssh 1.2.12 release by
.An Tatu Ylonen .
.An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos , Theo de Raadt
and
.An Dug Song
removed many bugs, re-added newer features and created OpenSSH.
.An Markus Friedl
contributed the support for SSH protocol versions 1.5 and 2.0.
